Before the mid-1960s, vacuum tubes were the technology used for audio amplification. Various vacuum tubes were developed for radio, television, radar, radio frequency (RF) power, audio, and specialized applications. Over several decades of design, with a limited selection of vacuum tubes, a few standard designs for audio amplification evolved. Vacuum tube power amplifiers consisted typically of a preamplifier stage, to increase a voltage signal, and an output stage, to provide power amplification. Output impedance of a vacuum tube power amplifier, without any feedback or transformers in the circuit, is limited by the characteristics of vacuum tube technology to tens or hundreds of ohms. Output transformers are usually used to lower this output impedance to provide good power transfer to low impedance loads, such as that provided in loudspeakers.
The semiconductor (transistor) revolution provided immediate advantages to the power amplifier industry over existing vacuum tube systems. Semiconductor systems are small and reliable, and they dissipate far less heat than vacuum tubes. Furthermore, transistors can be low voltage devices with low inherent impedances that eliminate the need for audio output transformers. These characteristics of semiconductors greatly reduce potential costs and eliminate the distortion effects and bandwidth limitations associated with transformers. The majority of systems and devices, which at one time relied on vacuum tubes, have been converted to semiconductors. Only a few vacuum tube types are manufactured and in regular use, predominantly in the high-end audio field.
Despite 35 years of transistor technology, and the simple task of amplifier design, there is no standardization within the industry. Audio experts have come to recognize that audio devices have inherent distortions to which the human ear is remarkably sensitive. The conventional measures of total harmonic distortion (“THD”) and frequency response have proven to be inadequate in comparing one amplifier to another.
Vacuum tube systems, with their obvious drawbacks of inefficiency, heat, unreliability, size, and high impedance, still command a strong presence in the high-end audio industry. Many listeners find vacuum tube power amplifiers to be more “transparent” than semiconductor systems, meaning the vacuum tube systems are less prone to the type of semiconductor distortions that change the original characteristics of a music signal. The survival of the vacuum tube amplifier defies the logic of conventional engineering measurements to this day.
For the past two decades, designers of high-end audio equipment have focused on the task of trying to get solid-state (e.g., transistor) amplifiers to sound like vacuum tube power amplifiers. These efforts have usually focused on the measurable distortion characteristics found in many of the older vacuum tube power amplifiers. The human ear finds even-ordered harmonics to be inherently of a musical nature and some favored vacuum tube power amplifiers are rich in these harmonics. Despite these efforts, no designer has yet succeeded in duplicating the quality of sound generated by vacuum tube power amplifiers, as evidenced by the wide variety of designs and systems that are to be found in the current market and the continued survival of vacuum tube products. The high-end audio music market has not shifted to one type of transistor circuitry as the best design.
The main focus of research for the audio industry has been directed toward circuitry. Presently, most high-end manufacturers of solid-state amplifiers recommend that their equipment should be “warmed up” before critical listening, but the manufacturers have not demonstrated that sound quality is related to thermal heating of solid-state components. The recommendation to “warm up” an audio system may originate from the classical vacuum tube systems in which “warm up” was necessary for operation. Most manufacturers need to keep the external case temperatures low for safety and reliability of audio appliances, and strive to keep the semiconductors within the audio appliances below sixty degrees Celsius.
Class A amplifiers have become popular in recent years due to their enhanced sound quality. The Class A amplifiers are designed for high output device currents, which improve linearity since the amplifiers are always conducting. In addition to increasing measured linearity, Class A amplifiers also elevate temperatures of the output devices, though this is not the stated purpose of the increased current. The consensus is that the higher the bias currents, as in the Class A amplifiers, the better the sound, since the circuit becomes more linear. As the current is increased in the output stage to increase this linearity, every effort is made to keep the output device temperature low with large heat sinks. Despite these improvements, they have not enabled solid-state audio systems to obtain the same “transparency” found in vacuum tube systems. Such Class A amplifiers fail to achieve this goal because they do not raise the temperature of the output devices sufficiently and make no attempt to raise the temperature of other semiconductor devices in the amplifiers, such as those found in the preamplifier stage.
Some of the best amplifiers have become passive heat managers. These amplifiers are provided in very large packages that maintain an elevated temperature. Present amplifiers typically maintain external heat sink temperature at no more than sixty degrees Celsius, and junction temperature at no more than approximately seventy degrees Celsius. The external heat sink temperature is required to stay low for safety.
Some amplifiers contain thermal monitoring or thermal control devices to determine the temperature of output devices. These thermal control devices are utilized to ensure that the output devices do not overheat and therefore are believed to contribute to system reliability. Other thermal control devices are designed to compensate for varying bias current caused by fluctuating temperature, to maintain signal gain relatively constant.
The present trend in the audio industry is to restrict temperatures of power devices. External heat sinks are used to maintain temperatures of power devices at about sixty-five degrees Celsius or lower, in order to keep products having such power devices therein, safe to touch. Presently, no one in the audio field has directly addressed the impact of thermal characteristics on sound quality enhancement.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.